The middle-ear includes a chamber that is bounded on one side by the eardrum and on another other side by the cochlea of the inner ear. In a healthy middle ear, this chamber is filled with air. A narrow passageway, called the eustachian tube, ensures that the static air pressure within the middle-ear chamber is equal to the ambient pressure.
Three very small bones, referred to as “ossicles,” provide a mechanical connection between the eardrum and the inner ear. The first and third ossicles are mechanically coupled to the eardrum and to the inner ear respectively. The second ossicle provides a mechanical linkage between the first and third ossicles.
The sensation of hearing results from sound waves that set the eardrum into motion. The motion of the eardrum, in turn, causes the three ossicles to vibrate. These three ossicles thus carry the vibrations across the air-filled chamber to the inner ear, where they are converted into an electrical signal that is recognizable by the brain. Because the transmission of sound across the air-filled chamber depends heavily on the vibration of the eardrum and the ossicles, it is essential that these structures remain free to vibrate at all times.
Certain diseases interfere with the free vibration of these structures. For example, otitis media, which is an inflammation of the middle-ear, can result in accumulation of fluid in the middle-ear chamber. If the eustachian tube cannot clear this fluid, the chamber will fill with fluid. Otitis media can also result in the build up of fibrous tissue within the chamber. Because this fibrous tissue is not a fluid, it cannot drain through the eustachian tube at all.
To appreciate the impact of fluid or fibrous tissue in the middle-ear chamber, it is useful to consider the operation of an air-filled kettledrum. When struck by a drumstick, the drum skin freely vibrates. This results in a deep and resonant sound. On the other hand, if instead of being filled with air, the kettledrum were to be filled with viscous fluid or cotton batting, and then struck with a drumstick, the resulting sound would be muffled. This muffling occurs, in part, because the material that now fills the kettledrum impedes the free vibration of the drum skin.
The presence of fluid in the middle-ear chamber also interferes with wave propagation in the inner ear. In normal operation, the vibration of the ossicles causes a pressure wave in the fluid filled cochlea of the inner ear. This wave propagates to the far end of the cochlea, activating hair cells and nerve endings as it does so. The propagation of this wave is made possible by pressure release at the other end of the cochlea. This pressure release is achieved by a flexible membrane, referred to as the “round window membrane,” that separates the fluid-filled interior of the cochlea from the normally air-filled middle-ear chamber. Any fluid collecting in the middle-ear chamber interferes with the pressure-release function of the round window membrane.
Nadol Jr. U.S. Pat. No. 5,356,430, the contents of which are herein incorporated by reference, teaches the placement of a gas-filled balloon within the middle-ear chamber in order to displace fluid and to allow motion of the round window membrane. However, the trilayer membrane of the balloon and the placement of the balloon within the middle-ear chamber near the round window membrane as taught in Nadol Jr. results in only limited improvement of middle-ear function.